In the first place, most chemists work with materials whose chemical formulae are fairly simple, and in which the ratio of the numbers of atoms of one kind to the numbers of atoms of another are small integers, or fractions of small integers. Among such compounds are NaCl, Al203 and Si02• By contrast, the geochemist often studies materials where one type of atom or ion can substitute for other types, and where the simple integral relationships no longer exist, e.g. (Ca2_ x Na.) Fe,-y-z Mgu Al,) (Si8_p Alp) 022 (OH)2_0 Fq), a general formula for hornblende (Unit 2), where elements within the rounded brackets can be present in a variety of relative proportions, i.e., x, y, z, etc., can have a range of values which are rarely whole numbers.
In the second place, the' chemist in his laboratory usually works with systems in which each phase is homogeneous, that is, any part of a particular phase has the same composition as any other part. Quite often, a chemist will employ mechanical methods (for example, stirring) to -ensure that this condition is met. Geochemists, however, are interested in systems such as the oceans, which are inhomogeneous and where conditions in one region may differ from those in another so that a reaction proceeding in one direction in one part of the system may proceed in the opposite direction elsewhere.
At the other end of the scale, geochernists deal with crystals, which are inhomo• geneous, differing markedly in composition and perhaps even in structural state, over distances measurable in microns only. For example, compositional variation from core to rim of a crystal indicates that physical and chemical conditions changed considerably during its growth.
Finally,• some of the important principles that chemists use when discussing chemical changes, such as the laws of chemical equilibrium, apply only to closed systems. This criterion is often not satisfied in geochemistry.
Surprisingly enough, however, the very size of geochemical systems can reduce the significance of all these qualifications. For instance, the changes produced in the concentrations of chemicals in large regions of the sea by the inflow and outflow of material are often negligible, so that in some respects the system behaves as though it were closed, and it is still helpful to apply the laws of chemical equilibrium to these regions. Moreover, we can monitor the behaviour of complex natural geochemical systems by using simple laboratory models which are familiar to the chemist, and which behave like the natural systems to a good approximation. We shall now recapitulate some of the principles which were used In the discussion of chemical reactions in SIOO, by considering a reaction of some geochemical importance, a solid-gas reaction.

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